The invention relates to an arrangement for measuring the moisture content of an aqueous substance by using microwaves produced by a transmitting arrangement formed by a signal source and a transmit aerial connected thereto. The aqueous substance is disposed in the field of radiation of the transmit aerial a receive aerial and receives the wave transmitted through the substance. An evaluation arrangement is connected to the receive aerial.
Several arrangements for measuring the moisture content of an aqueous substance by means of microwaves are known. The "Proceedings of the Microwave Power Symposium", Toronto, 9-12 June 1980, pages 193-195 discloses an arrangement in which a coaxial aerial ends in a tube in which the substance to be measured is present. The coaxial aerial is connected to a measuring arrangement which determines the reflection factor of the coaxial aerial, which depends on the permittivity and consequently on the water content of the substance present in the tube.
This measuring arrangement has the disadvantage that a very accurate and consequently costly determination of the permittivity at the end of the coaxial aerial is required. In addition, only that portion of the substance to be measured which, in the tube, is in the immediate vicinity of the coaxial aerial influences the measured reflection factor. Consequently, the measuring arrangement is not suitable for use with non-homogeneous substances.
The "Journal of Microwave Power", Volume 14, 1979, No. 4, pages 363 to 365 discloses a strip-line sensor for measuring moisture by means of microwaves. A strip line provided on a substrate is enveloped by the substance to be measured which is penetrated by the stray field of the strip line. The moisture contained in the substance influences via the stray field the attenuation of the strip line which is connected to a measuring arrangement for attenuation measurement.
This arrangement, as is also the case for the coaxial aerial, has the disadvantage that the stray field is only present in the immediate vicinity of the strip line and that consequently only a small portion of the substance to be measured affects the attenuation of the strip line. This arrangement is therefore also not suitable for measurements on larger quantities of non-homogeneous substances. Moreover, attenuation measurements have the drawback that they are sensitive to interferences by standing waves, so that measurements made with the described arrangement become inaccurate.
In practice it is difficult to determine high moisture contents by means of microwaves as the highly aqueous substances on which the measurements are to be effected often consist of a small quantity of a material having a low permittivity, typically a value between approximately 2 and approximately 5, and a large quantity of water having a permittivity of approximately 80. The resultant permittivity of the aqueous substance then deviates only little from the permittivity of water, and changes in the water content affects the propagation properties of the substance, and consequently the measurements, to a very limited extent only.